1. Field of the Invention
The present invention relates to a duobinary optical transmission apparatus using a duobinary optical transmission technique.
2. Description of the Related Art
Typically, a Dense Wavelength Division Multiplexing (DWDM) optical transmission system transmits an optical signal comprised of a plurality of channels with different wavelengths using a single optical fiber in such a way that it enhances transmission efficiency. As such, the DWDM optical transmission system has been widely used in a superhigh-speed Internet network requiring a large amount of data transfer due to its ability to transmit optical signals irrespective of the transfer rate. Recently, systems capable of transmitting more than 100 channels via a single optical fiber using the DWDM optical transmission techniques have been produced commercially, and a new system capable of transmitting more than 200 channels each having a transfer rate of 40 Gb/s (Gigabit per second) simultaneously to accomplish a transfer rate of more than 10 Tbps is being intensively researched.
Although new system accommodates rapidly increasing data traffics as well as transfer requests for high-speed data of more than 40 Gbps, a conventional optical intensity modulation method using a NRZ (Non-Return to Zero) method has a limitation in increasing the transfer quantity as an abrupt interference and distortion between channels occurs in a prescribed zone, which is less than a channel interval of 50 GHz. In addition, the DC frequency components of a conventional binary NRZ transmission signal and the high-frequency components spreading in a modulation process cause nonlinear characteristics and dispersion while the signals are propagated along the optical fiber medium, thereby limiting a transmission distance at a high-speed transfer rate over 10 Gbps.
Currently, an optical duobinary technique has been intensively researched as a new optical transmission technique for obviating the transmission distance limitation caused by the chromatic dispersion. The optical duobinary technique has an advantage in that it reduces a width of a transmission spectrum much more than a general binary transmission method. The transmission distance in a dispersion limitation system is inversely proportional to a square of the transmission spectrum bandwidth. As such, in the case where the transmission spectrum bandwidth is reduced by half, the transmission distance increases by four times. Moreover, a carrier wave frequency is suppressed in a duobinary transmission spectrum such that limitations in the output optical power caused by the Brilloum Scattering stimulated in the optical fiber are reduced.
FIG. 1 is an exemplary view illustrating a block diagram of the conventional duobinary optical transmission apparatus, and FIGS. 2a˜2c are views illustrating eye-diagrams of output signals at A, B and C nodes shown in FIG. 1, respectively.
Referring to FIG. 1, a conventional duobinary optical transmission apparatus includes a Pulse Pattern Generator (PPG) 10 for generating electric pulse signals using a NRZ method; a pre-coder 20 for encoding the electric pulse signals; drive amplifiers 30 and 31 for amplifying a two-level signal generated from the pre-coder 20; LPFs (Low Pass Filters) 40 and 41 for converting the amplified two-level signal into a three-level signal and reducing a bandwidth of the three-level signal; a laser source 50 for generating a carrier wave; and, a Mach-Zehnder-interferometer-type optical intensity modulator (MZ MOD) 60 for receiving the amplified three-level signal and converting the carrier wave into a two-level optical signal.
In operation, the NRZ-type electric pulse signals generated by the PPG 10 are encoded as binary signals of 0 or 1 by the pre-coder 20. An output eye-diagram at a node A is shown in FIG. 2a. The two-level binary signal generated from the pre-coder 20 is amplified by the drive amplifiers 30 and 31, then transmitted to the LPFs 40 and 41. The LPFs 40 and 41 each have a bandwidth corresponding to about ¼ of a clock frequency of the two-level binary signal. This excessive bandwidth restriction causes interference between codes, and the two-level binary signal is converted into a three-level duobinary signal because of this interference between codes. An output eye-diagram at a node B is shown in FIG. 2b. 
Note that the amplified three-level duobinary signal is used as a driving signal of the Mach-Zehnder-interferometer-type optical intensity modulator (MZ MOD) 60. The carrier wave generated from the laser source 50 modulates its own phase and its own optical intensity according to the driving signal of the MZ MOD 60, and is thereby generated as a two-level optical duobinary signal. An output eye-diagram at a node C is shown in FIG. 2C. Referring back to FIG. 1, a reference character Q indicates the inverted signal of a signal Q, and the signals Q and Q are respectively transmitted to a positive electrode (+) and a negative electrode (−) of the MZ MOD 60 with a dual electrode structure.
The MZ MOD 60 is classified into a Z-cut type MZ MOD and an X-cut type MZ MOD according to its own electrode structure. As shown in FIG. 1, the Z-cut type MZ MOD having a dual electrode connects its own both arms to the drive amplifiers 30 and 31 and the LPFs 40 and 41 in such a way that a three-level electric signal is applied to each of both electrodes of the Z-cut type MZ MOD. Although the X-cut type MZMOD having a single electrode is not shown in the drawings, it connects its own one arm to a drive amplifier and a LPF in order to transmit a three-level signal to one electrode.
FIGS. 3a˜3b are views illustrating the eye-diagrams of output signals transmitted via a single mode optical fiber using a duobinary optical transmission apparatus. Referring to FIGS. 3a˜3b, it is noted that eye-diagrams are relatively maintained clearly in the range of a transmission distance from 0 km (shown in FIG. 3a) to 160 km (shown in FIG. 3b).
However, the aforementioned conventional duobinary transmission technique generates a three-level electric signal using a LPF, such that a difference is generated on the basis of dependency of transmission quality corresponding to the transmission characteristics of a LPF and the length of a Pseudo Random Bit Sequence (PRBS), thereby causing a serious problem in an overall system. This jitter problem is generated in the Z-cut type or X-cut type conventional structure. Accordingly, a dependency of such signal patterns provides a limitation in a real optical transmission operation.
Furthermore, in case of using the Z-cut type MZ MOD having a dual electrode, the output signals Q and Q are respectively input to LPFs of both arms of the Z-cut type MZ MOD, amplified by amplifiers, and then transmitted to both terminals of the Z-cut type MZ MOD. In this case, the Z-cut type MZ MOD must perform a push-pull operation, where electric signals applied to both terminals of the Z-but type MZ MOD must have different polarities having a difference 180° therebetween. In the case where the electric signals do not have such different polarities, it is necessary to adjust a phase using a delay line. Thus, two LPFs and two amplifiers must have the same characteristics. However, it is impossible to manufacture two LPFs and amplifiers with same characteristics as a tiny difference between the two LPFs or the two amplifiers is unavoidable during the manufacturing process.
In addition, a Bessel-Thomson type LPF widely used as a LPF is very expensive, thereby increasing the cost of production of an overall optical transmission apparatus.